Equilibria and ammonia
Students mainly experience chemical reactions that appear to go to completion. When they meet a reaction that does not go to completion but which has a reverse reaction occuring they find the concept difficult to understand.
One major misconception students have about equilibrium is that they think equilibrium positions are fixed and once achieved there is no movement of particles between the two 'sides' i.e. they believe that equilibria are static not dynamic.
Le Chateliers Principle is a way of predicting changes to an equilibrium position under some circumstances but is generally wrongly applied due to the misunderstanding about the equilibrium position.
Rate and equilibria are often confused because students think that the rate of one reaction may change while the other slows or remains constant. They have not grasped that rate applies to the system as a whole.
It is important to use a wide range of reversible reactions to help get these ideas across to the students. A variety of simulations and modelling activites are available to support students which are explained in the following list.
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance
Links and Resources
Before showing this video you will need to introduce and explain Le Chatelier's Principle - that a system at equilibrium will change to oppose the change taking place. i.e. if the concentration of the reactants increases the equilibrium will move to the side of the products (that is to the right).
There are two video clips which demonstrate the effects of changing temperature and pressure on the equilibrium between dinitrogen tetroxide and nitrogen dioxide. It is a very visual demonstration due to the colour change that takes place and it also helps students to understand that gaseous reactions need to take place in a closed container.
You will need to provide a balanced equation for the reaction and explain that nitrogen dioxide is a brown gas and it becomes colourless dinitrogen tetroxide. It is important to point out that there is I molar gas volume on the dinitrogen tetroxide side and 2 molar gas volumes on the nitrogen dioxide side of the reaction, which is why pressure has an effect on the position of equilibrium.
The third clip which show the formation of dinitrogen trioxide is not really suitable for GCSE level.
This is an important activity as it demonstrates what is happening in a system that has reached a dynamic equilibrium. It helps to overcome the common misconception that there are no movement of particles once the system is at equilibrium.
An alternative activity is the use of double sided cards (Frogs and Princesses). In this activity each group of students have 24 double sided cards which are placed on two A3 sheets (this represents the reaction boundary). Students start with princesses facing up and turn them over until the frogs side is showing. This models a reaction that goes to completion. They repeat the activity, this time stopping when about 66.6% complete. At this point one student represents the forward reaction while another represents the backward reaction, and they continue turning over the double sided cards while monitoring the overall number of frogs and princesses showing face up. This should help get across the idea of a dynamic equilibrium. This activity can be extended to discuss what happens when the boundaries are halved or doubled, or the number of reactants are increased.
A alternative model is the use of escalators - this can be used to model a dynamic equilibrium and the fact that the equilibrium can be at any position (not necessarily in the middle). Imagine someone was walking up the down escalator and reached the middle but then appeared to have stopped. They would actually still have to be moving to remain at that position, which represents a dynamic equilibrium i.e. still moving even though it appears to be stopped. This model can also be used to explain that the eqilibrium position is not always in the middle but can be anywhere in between.
Reactions that have equilbrium positions far to the left often need catalysts to help them proceed. Those that are far to the right are almost to completion.
This provides some useful background information about Fritz Haber and can be used to discuss the implications of the Haber Process. It is an opportunity to conduct an activity which addesses the spiritual, moral, social and cultural aspect of the national curriculum.
Students can work in groups, provided with information about Fritz Haber from which to make notes to use in a class discussion. Each group could be allocated a particular point of view to use when reading the materials and making notes i.e. a different coloured De Bono's hat. Once the groups have made their notes they nominate a spokesperson to take part in a small discussion (with representatives from other groups). This is watched by the whole class, who observe different aspects of the discussion and provide feedback. This activity is a modification of a Socratic discussion which is popular in school English departments.
Students might be interested to note that a career in chemical engineering or firework design and manufacture (page 8) would make use of the knowledge gained from understanding about equilibriaum and reaction rates.
In chemical engineering it is important to ensure the correct conditions for any given reaction to get the maximum yield at minimal cost, as in the Haber Process.
In firework design it is essential to get the correct mixture for effective and safe manufacture of fireworks!